1. Field of the Invention
The present invention relates to a method and apparatus for data communication for sending and receiving data such as character code data, image data or the like via a communication network, particularly suitable for application to data transmission via an international communication network.
2. Related Background Art
Among types of data communication apparatus, facsimile apparatus for example are known for sending and receiving image information. In a case where a conventional facsimile apparatus calls a receiving station in an automatic sending mode, it is possible for the facsimile apparatus to perform necessary procedures by confirming by itself that the apparatus operates as a calling station. However, for the case that a call is made by a telephone set prior to sending image information with the facsimile apparatus, a conventional facsimile apparatus is not constructed in such a way that it can confirm or detect by itself that it is operating as a calling station.
Alternatively, in the case where a conventional facsimile apparatus is previously set in an automatic receiving mode, it is possible for the facsimile apparatus to perform necessary procedures by confirming by itself that the apparatus operates as a receiving station. However, for the case that the facsimile apparatus is set in a manual receiving mode, a conventional facsimile apparatus is not constructed in such a way that it can confirm to detect by itself that it is operating as a receiving station. In this case, only an audible ringing of the telephone is generated.
By transmitting an echo suppressor function termination tone to a receiving station prior to commencing necessary data communication over an international communication network, it is possible to send a control signal from a called station to a calling station during data transmission from the calling station to the called station (i.e., so-called full duplex operation).
However, on the contrary, a full duplex operation cannot be achieved even if a control signal is sent from a calling station to a called station during data transmission from the called station to the calling station, even if an echo suppressor function termination tone was sent prior to commencing international communication. Therefore, if a "continuous error retransmission" is intended in the latter case (during data transmission from a called station to a calling station), the image information sending station cannot detect a control signal (e.g., a NACK signal which is sent when a reception error occurs) and data communication becomes impossible. This is a serious disadvantage associated with a conventional facsimile apparatus.
The "continuous error retransmission" here mentioned means an error retransmission system in a full duplex operation. More in detail, according to such system, when an error is detected at a receiving station during transmission of image information from a sending station, the receiving station sends a NACK signal to the sending station to suspend transmission of image information. Thereafter, the image information is retransmitted starting from for example the address frame line where the error was generated.
In the case of the facsimile apparatus, for example, the echo suppressor function is stopped upon sending a called station identification signal (CED) to use the telephone network in a full duplex mode. However, even if the echo suppressor function is stopped, this stoppage is released if, for a period longer than a predetermined time interval, signals are not being sent.
For instance, if it takes a long time for the operator at a sending station to depress a sending start button after hearing a CED signal, response signals NSS/TSI/DCS cannot be sent from the sending station in response to first, initial identification signals NSF/CSI/DIS sent from the receiving station. Consequently, the receiving station must send the initial identification signals once again. However, since signals are not being sent for the relatively long period between the first and second initial identification signals, the stoppage of the echo suppressor function is released.
In addition, if a receiving station cannot receive signals of DCS group, or page interruption signals such as EOP, MPS, EOM or the like, the stoppage of the echo suppressor function is also released in this case.
Releasing the stoppage of the echo suppressor function makes it impossible to communicate in a full duplex mode. Accordingly, the continuous error retransmission using the network in a full duplex mode becomes also impossible.